The Effects of Dopants on the Cu–ZrO2 Catalyzed Hydrogenation of Levulinic Acid

Catalytic hydrogenation of levulinic acid to form γ-valerolactone was studied over Cu–ZrO2 catalysts doped with metal oxides from the first-row transition metals. The Cu–ZrO2 material was prepared by oxalate gel coprecipitation, and dopants were added by an incipient wetness approach. The addition of 1% Mn into Cu–ZrO2 significantly increases the yield of γ-valerolactone, and the catalytic activity of Mn/Cu–ZrO2 was found to be 1.6 times higher than that of the undoped Cu–ZrO2 catalyst. Catalyst characterization suggests that the Mn dopant improves the dispersion of Cu on the surface of ZrO2. Kinetic studies show that the reaction order with respect to the substrate concentration is approximately zero. However, the order of reaction with respect to the partial pressure of H2 is different for the Mn/Cu–ZrO2 and Cu–ZrO2 catalysts. Comparison of reaction products from reactions carried out in H2O and D2O solvents using 1H NMR and 13C NMR show that there is a pre-equilibrium keto–enol isomerization step under our reaction conditions. DFT calculations show that the enol isomers have a higher affinity for the Cu surface, which may improve the availability of the substrate in the hydrogenation step of the reaction.